Standard AR-15 rifles accept detachable magazines of widely varying capacities, and have a pistol grip that protrudes beneath the stock. AR-15 rifles are highly configurable and customizable. They are commonly fitted with several accessories such as bipods, folding or collapsing stocks, threaded barrels for the attachment of a flash suppressor, and a rail system for the attachment of vertical grips, flashlights, laser sights, telescopic sights, etc.

The AR-15 consists of separate upper and lower receiver assemblies, which are attached with two through-pins and can be quickly interchanged with no tools. Under U.S. firearms laws, only the lower receiver is considered a weapon and subject to purchasing restrictions. The upper receiver assembly is simply considered a part, and may be freely purchased and mail-ordered in most locations. This is a very attractive feature for enthusiasts, who often purchase a number of upper receivers (often in different calibers) and interchange them with the same lower receiver. However, one must be thoroughly familiar with firearms laws before doing this as it is possible to make an illegal configuration. For example, an 11" barrel with only a pistol grip is a legal handgun in most locations. It should be noted that the ATF enforces the doctrine of "once a rifle, always a rifle"; as such, an AR-15 pistol must be constructed using a lower receiver that has never been constructed as a rifle or described on an ATF form 4473 as a rifle. Adding a shoulder stock to an AR-15 with a barrel shorter than 16" would constitute constructing a Short-Barreled Rifle (SBR) under NFA rules. Constructing an AR-15 pistol using a lower receiver which had already been constructed as a rifle or described as such on an ATF form 4473 would also constitute the construction of an SBR. It is a felony to assemble, possess, or transfer such a weapon without obtaining ATF approval and the corresponding $200 tax stamp.

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The AR-15 is based on the 7.62 mmAR-10, designed by Eugene Stoner of the Fairchild ArmaLite corporation.[9] The AR-15 was developed as a lighter, 5.56 mm version of the AR-10. The "AR" in AR-15 comes from the ArmaLite name. ArmaLite's AR-1, AR-5, and some subsequent models were bolt action rifles, and there are shotguns and pistols whose model numbers also include the "AR" prefix.[9]

Colt AR-15 Sporter SP1 Carbine

ArmaLite sold its rights to the AR-10 and AR-15 to Colt in 1959. Colt marketed the AR-15 rifle to various military services around the world, including the U.S. Air Force, Army, and Marine Corps. The AR-15 was eventually adopted by the United States military under the designation M16. However, Colt continued to use the AR-15 trademark for its semi-automatic variants (AR-15, AR-15A2) which were marketed to civilian and law-enforcement customers. The original AR-15 was a very lightweight weapon, weighing less than 6 pounds with empty magazine, though later heavy-barrel versions of the civilian AR-15 can weigh upwards of 8.5 lbs.[11][12]

Today the civilian-model AR-15 and its variations are manufactured by many companies and have captured the affection of sport shooters and police forces around the world due to their accuracy and modularity. (Please refer to the M16 for a more complete history of the development and evolution of the AR-15 and derivatives.)

The trademark "AR15" or "AR-15" is registered to Colt Industries, which maintains that the term should only be used to refer to their products. Other manufacturers make AR-15 clones marketed under separate designations, although colloquially these are sometimes also referred to by the term AR-15.

Semi-automatic AR-15 for sale to civilians and fully automatic versions for sale to law enforcement and military customers, though nearly identical in appearance, are quite different internally. The hammer and trigger mechanisms are of a slightly different design. The bolt carrier and internal lower receiver of semi-automatic versions are milled differently, so that the firing mechanisms are not interchangeable. This was done specifically to satisfy BATF requirements that civilian weapons may not be easily convertible to fully-automatic. Despite this, through use of a "Drop In Auto Sear" or "lightning-link," conversion to full automatic is very straightforward (sometimes requiring slight modification to the bolt carrier).[13][14] Such modifications, unless using parts made prior to 1986, are illegal. An illegally converted fully-automatic AR-15 was used in the North Hollywood shootout.

Automatic variants have a three-position rotating selective fire switch, allowing the operator to select between three modes: safe, semi-automatic, and either automatic or three round burst, depending on model. Civilian AR-15 models do not have three-round burst or automatic settings on the fire selector. In semi-automatic only variants, the selector only rotates between safe and semi-automatic. Due to this, weapons modified to fully automatic using a lightning-link are capable of fully automatic fire only unless a fully-automatic fire select switch is substituted.[15]

The mechanism of operation for the rifle is known as direct gas impingement. Gas is tapped from the barrel as the bullet moves past a gas port located above the rifle's front sight base. The gas rushes into the port and down a gas tube, located above the barrel, which runs from the front sight base into the AR-15's upper receiver. Here, the gas tube protrudes into a “gas key” (bolt carrier key) which accepts the gas and funnels it into the bolt carrier. This movement of gas into the bolt carrier forces the bolt and carrier backward in line with the stock of the rifle. As the bolt carrier moves toward the butt of the gun, the bolt begins to turn and unlock from the barrel extension. Once the bolt is fully unlocked it begins its rearward movement along with the bolt carrier. The cam pin is responsible for the bolt's rotation as it follows a groove cut into the carrier that twists and forces the bolt to unlock. Once the bolt is unlocked, the bolt carrier and bolt continue to move toward the butt of the gun and the empty case is extracted and ejected out the ejection port on side of the upper receiver.

Behind the bolt carrier is a plastic or metal buffer which rests in line with a return spring that pushes the bolt carrier back toward the chamber. A groove machined into the upper receiver traps the cam pin and prevents it and the bolt from rotating into a closed position. The bolt's locking lugs then push a fresh round from the magazine which is guided by feed ramps into the chamber. As the bolt's locking lugs move past the barrel extension, the cam pin is allowed to twist into a pocket milled into the upper receiver. This twisting action follows the groove cut into the carrier and forces the bolt to twist and “lock” into the barrel’s extension.

The AR-15 rifle is available in a wide range of configurations from a large number of manufacturers. These configurations range from short carbine-length models with features such as adjustable length stocks and optical sights, to heavy barrel models. Some of the most popular manufacturers are ArmaLite, Bushmaster, and DPMS[citation needed].

When installing a new complete upper receiver, particularly one designed to handle a different caliber of ammunition (i.e. other than .223 Remington or 5.56 x 45 mm NATO), some modification to the contents of the lower receiver may also be required, depending on the particular conversion. For example, a conversion to 9 mm typically would involve the installation of a magazine well block (to accommodate a typical 9 mm magazine, such as Uzi or Colt SMG), replacing the .223 hammer with one designed for 9 mm ammunition, and depending on the original stock, replacing the buffer, action spring and stock spacer with those designed for the new 9 mm AR-15 configuration.

Early models had a 1:14 rate of twist for the original 55 grain (3.6 g) bullets. This was changed to 1:12 when it was found that 1:14 was insufficient to stabilize a bullet when fired in cold weather. Most recent rifles have a 1:9 or 1:7 twist rate. There is much controversy and speculation as to how differing twist rates affect ballistics and terminal performance with varying loads, but heavier projectiles tend to perform better with faster rifling rates.[citation needed] Additionally, the various non .223 / 5.56 calibers have their own particular twist rate, such as 1:10 for 6.8x43mm SPC and 7.62x39mm, and 1:12 for .308 Winchester.

Standard issue magazines are 20 or 30 round staggered-column magazines, traditional box magazines also exist in 40 and 45 round capacities, and usable magazines have been constructed from a variety of materials including steel, aluminum, and high-impact plastics. Drum magazines with 90 and 100 round capacities also exist, such as Beta C-Mags. Low-capacity magazines, usually of a 5 or 10 round capacity, are available to comply with some areas' legal restrictions, hunting and because larger magazines can inhibit shooting from a benchrest.

In the United States, variants with certain features such as collapsible stocks, flash suppressors, and bayonet lugs were prohibited for sales to civilians during the period 1994-2004 by the Violent Crime Control and Law Enforcement Act of 1994, under the provision known as the Assault Weapons Ban. Included in this was a restriction on the pistol grip that protrudes beneath the stock, which was considered an accessory feature under the ban and was subject to restrictions. Some rifles were manufactured with a grip not described under the Ban installed in its place. Those AR-15s that were manufactured with those features were stamped, "Restricted Military/Government/Law Enforcement/Export Only" as well as the accompanying high capacity magazines. Since the expiration of the Federal AWB in September 2004,[16] these features are now legal in most states.[17]

The 2000 Assault Weapons ban in the state of California sparked a renewed interest in the AR-15 rifle. It is estimated that some 70,000 California Legal AR-15s are in existence in that state.[18] Adding the upper receiver of a standard AR-15 or equivalent with an AR-15 equivalent lower receiver which has not been specifically banned by statute or regulation, and that has a fixed 10 round magazine will render the firearm "California legal." In such a configuration, the user could add otherwise prohibited features such as a telescoping stock and pistol grip. The magazine is not detachable, so to load the rifle the shooter must pull the rear takedown pin, hinge the upper receiver on the front pivot pin, and load the now exposed magazine either with a stripper clip or by hand, then close. Popular lower receivers for this purpose are manufactured by Bushmaster, Stag Arms, Fulton Armory, Dane Armory, Mega, and Ameetec. By California law if the magazine requires a tool to remove it, that changes the classification of the firearm. A tool called the "Bullet Button" is gaining in popularity: the bullet button works by replacing the magazine release button with a hollow shell that protrudes a short distance from the lower; the shooter must then push the inset pin to activate the mag release, doing so requires a tool e.g., a bullet, hence the name. Stag makes a lower receiver called the STAG-15 which is considered an "off-list" receiver by the CA DOJ and is legal. As of December 2006, Doublestar, Stag Arms, CMMG, Spikes Tactical, and MEGA all qualify as "off-list" lowers in the state of CA. There is also one model made by Colt, the CAR-A3 HBAR Elite, that was never banned by name, and thus still legal to own in California provided it has the correct configuration. This receiver can be made into a full rifle if the following requirements are met: the receiver has a fixed magazine with no more than 10 cartridges — in which case the rifle may have pistol grips, folding or collapsing stocks, etc.; or, the receiver may have a detachable magazine but may not possess any sort of attachment such as pistol grips, folding or collapsing stocks, etc.

The Government of Canada classifies the AR-15 (and its variants) as a restricted firearm. For a citizen to lawfully own an AR-15, he or she must first pass a "Canadian Non-Restricted Firearms Safety Test", and then a "Canadian Restricted Firearms Test". This allows the applicant to obtain a Possession and Acquisition Licence (PAL) valid for restricted firearms.[19][20] With the introduction of strict gun control measures by Prime Minister Jean Chretien (Bill C-68), the AR-15 had originally been intended to be classified as Prohibited, making it all but impossible to privately own one, however due to the presence of nationwide Service Rifle target shooting competitions, the AR-15 was granted a sporting exception.

As with all Restricted firearms (including all pistols, some shotguns, and some rifles) AR-15s are only allowed to be shot at certified firing ranges. In order to legally own and transport a Restricted firearm, a citizen must also register their firearm with the Royal Canadian Mounted Police and must apply for an Authorization to Transport (or ATT) permit from the RCMP Chief Firearms Officer (CFO) for their province. Additionally, the firearm must be unloaded, deactivated by a trigger or action lock, and be in a locked, non-transparent container during transport[21]

The issuance of ATTs varies considerably from Province to Province, and is generally reflective of a particular province's political and social levels of acceptance toward gun ownership. For example, in Alberta, where firearms ownership is widely accepted, generally a single ATT is promptly issued that allows citizens to transport firearms to border crossings, gunsmiths, and shooting ranges. Conversely, Quebec and Ontario, where firearms ownership is generally not accepted, the CFOs do not readily issue ATTs and generally require separate ones for transportation for any reason.

As with all full-bore semi-automatic rifles, AR15's are classed as a Section 5 weapon (ie all but impossible for a private citizen to posess) however there are some companies who manufacture UK legal AR15 variants in a manual straight pull single shot configuration which is able to be held on a Section 1 Firearms Certificate. There are however no restrictions on magazine capacities in the UK.

With the plethora of manufacturers of complete weapons and aftermarket barrels, there is a potential hazard associated with chamber specifications. Both civilian (SAAMI) specification .223 Remington and 5.56 mm NATO are available. Though both chambers typically accept both types of ammunition, the firing of military specification ammunition in civilian specification chambers can produce chamber pressures greater than the barrel is designed to handle. The most common result of firing military 5.56x45mm ammunition in a .223 Remington chamber is that the primer can be forced out of the case by chamber pressure, often resulting in the primer becoming lodged somewhere in the action of the rifle, and disassembly of the rifle is often necessary to remove the jammed primer.[22] Military specification chambers typically have a more open throat area producing less pressure and can handle both types of ammunition.

A few AR15 manufacturers incorporate the use of a hybrid chamber specification known as the Wylde chamber. Designed by and named after Bill Wylde, this chambering was created for High Power shooters after the 80 grain .224" bullets became popular. While the Wylde chamber allows for optimal seating depth of 80 grain bullets over .223 Remington and 5.56 NATO, it is capable of accepting both ammunition types. The Wylde chamber is used by a few manufacturers who sell "National Match" configuration AR-15 rifle, barrels, and upper receivers.

The type of chamber, manufacturer, and rifling twist in inches is typically found stamped into the barrel in front of the front sight assembly.

An additional point of concern in the design is the inertial firing pin. A lightweight firing pin rides in a channel inside the bolt unrestrained. When the bolt locks forward during loading, the firing pin typically rides forward and impacts the primer of the chambered round. In military specification ammunition and quality civilian ammunition, this is not normally enough to fire the round and only leaves a small "ding" on the primer. With more sensitive primers or improperly seated primers, this can cause a slamfire during loading.[23]